This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 2 |
| 3 | -4 |
| 4 | 16 |
| 5 | -80 |
| 6 | 432 |
| 7 | -2304 |
| 8 | 10944 |
| 9 | -35328 |
| 10 | -74112 |
| 11 | 2736384 |
| 12 | -30853632 |
| 13 | 238663680 |
| 14 | -1247457280 |
| 15 | 2201247744 |
| 16 | 32530722816 |
| 17 | -320650199040 |
| 18 | 156266184704 |
| 19 | 18314630348800 |
| 20 | -20667999748096 |
| 21 | -3428200020508672 |